Recently, great progress has been made in the development of a fuel cell, which is expected as one of the most effective solutions to the environmental problems. Particularly, a fuel cell using solid polymeric film as electrolyte has been the mainstream in the current fuel cell development since the solid polymeric film having the operation temperature of as low as 80° C. is easy to be treated. This fuel cell uses hydrogen fuel, and thus requires a gas sensor for detecting hydrogen as the safety measures for preventing hydrogen leak.
A gas sensor having been currently proposed utilizes such characteristics of hydrogen that its thermal conductivity is extremely larger than those of other gases and detects variations in thermal conductivity due to the existence of hydrogen from temperature variations of a heating element. When hydrogen exists in the air, for example, larger quantity of heat is robbed from the heating element compared with the case where only the air exists. The temperature of the heating element therefore varies in accordance with the concentration of hydrogen, and those variations in temperature are electrically detected as changes in the resistance value of a temperature detecting element.
The heating element of the gas sensor, which is also used as the temperature detecting element, is formed by a platinum thin-film resistor. The platinum thin-film resistor, which has a thin-film structure, is manufactured using semiconductor micro machining technique, and is thus capable of producing minute heating elements. Thus, the power consumption decreases, and the response speed of the gas sensor increases. The gas sensor having this structure has been disclosed in JP-A-8-101156, for example.
When this type of gas sensor is used for detection of a hydrogen leak, a problem arises if moisture is contained in hydrogen as detection target gas. The resistance value of the heating element varies in accordance with the concentration of hydrogen if no moisture is contained. However, the resistance value also varies with the existence of moisture if it is contained, and it is therefore impossible to make distinction between changes caused by hydrogen, by moisture, and by coexistence of those.
In order to overcome this problem, the conventional gas sensor described above varies current which flows in the heating element formed by the platinum thin-film resistor. In this structure, the output voltage of the heating element changes in accordance with the degree of reaction, and the voltages at the both ends of the heating element obtained when respective levels of current flow therein are substituted in estimate equations established in advance and the equations are simultaneously calculated. Then, the quantities of the atmospheric gases, that is, the concentrations of the respective gases are calculated from the solutions of the estimate equations.
Basically, the gas concentrations of a plurality of constituents can be obtained by this method. However, a problem occurs when hydrogen substantially saturated with moisture leaks in the atmosphere at a temperature of nearly 80° C. in such a case as leakage detection from a fuel cell. When variations in the thermal conductivities of the respective gas constituents are expressed by linear equations or in such applications where those variations are detected only in the range of linear equations, the gas concentrations can be calculated using Chebyshev's orthogonal polynomial. In case of the fuel cell, however, it is estimated that a larger quantity of vapor than that of hydrogen is contained. In this condition, the thermal conductivities of the mixture family have non-linear characteristics which are always quadratic or have higher degrees, and rise as humidity increases, exhibit a peak, and then decrease. Thus, complicated calculations are required when solutions are obtained only from the simultaneous estimate equations. Moreover, since a plurality of solutions correspond to humidity, it is impossible to determine one value as humidity. As a result, it is also impossible to obtain one value as concentration of hydrogen.